Modern transducer arrays for sound and ultrasound generation consist of an orderly arrangement of identical or approximately identical transmitting elements which are designed to operate in either the longitudinal mode or a transverse mode. Where possible, the lateral or transverse dimensions of an individual transducer are selected to be approximately equal to or less than half the wavelength of sound of designed frequency in the intended acoustic medium (λm/2) to yield a strong and focused main acoustic beam while avoiding grating lobes. In addition, the strength of side lobes is kept low via beamforming and/or beam shading technique.
FIG. 1 shows an example of such a transmitting array 100 used in a side-scan or synthetic aperture sonar for underwater imaging purposes in which discrete rectangular elements 102 are spaced about λm/2 apart in longitudinal and crosswise directions, where λm is the wavelength of sound or ultrasound of designed central frequency in water. Using discrete transducer elements 102 in an array enables the main acoustic beam to be steered electronically via the elemental phasing technique. The control of the elemental separation in the array is important in such an application.
FIG. 2 illustrates an example of a 1D phased array medical transducer 200 used for both ultrasound transmission and reception in medical imaging reported in ‘Forty-channel phased array ultrasonic probe using 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 single crystal’ by Saitoh et al. The array 200 demonstrates discrete sliver elements spaced about λm/2 apart in the longitudinal direction, where λm is the wavelength of ultrasound of designed central frequency in human tissues. In such a device, top and bottom electroded faces 210 and 212 respectively are bonded onto a piezoelectric active material 204, which in turn is bonded onto a backing material 214 while an acoustic matching layer 206 is bonded onto the top electrode layer 210. The active material 204 is then diced into discrete silver elements together with the top electrode layer 210 and the matching layer 206. Finally, an acoustic lens 208 is placed over the acoustic matching layer 206 as shown in the figure.
Similar transducer arrays are also used for sound and ultrasound reception. While it is common for such receiving arrays to operate in off-resonance mode, they exhibit enhanced receiving sensitivity for sounds of frequencies comparable to the resonance frequencies of the constituting elements in the receiving mode.
Modern ultrasonic transmitting elements are driven by lead zirconate titanate (PbZr0.52Ti0.48O3 or PZT) polycrystalline ceramics. For direct-drive, piston-less transmitting elements, rectangular, rod or tube shaped ceramics are commonly used. These ceramics are poled across two opposite faces which act as electrodes to attend the desired piezoelectric properties. Conventionally, this poling direction is designated as the 3− direction.
In a longitudinal (33) mode operation, an active material is activated along the poling (3−) direction which is also the acoustic beam direction. An example of a transducer element 300 operating in the longitudinal (33) mode is provided in FIG. 3. In this figure, an active element 302 is provided with a backing material 304. The backing material 304 is a soft and high-damping backing material, which has the effect of decreasing ringing of the active element 302 for improved axial resolution when short pulse length signal is used. It also helps to widen the bandwidth of the transducer 300 at the expense of acoustic power. The shaded top face 306 and the bottom face 308 opposite to the top face 306 of the active element 302 act as electrodes. In such a design, the active material 302 in the activated direction is typically of half-wavelength (λc/2) in dimension, where    λc=v33D/f,    v33D is the sound velocity of the ceramic or single crystal active material along the activated longitudinal (3−) direction;    f is the desired sound frequency;    D indicates that the active material 302 is under a state of constant electrical displacement.
In the conventional transverse (31) mode operation, the active material is excited in resonance in one of the two lateral or transverse directions, which is also the acoustic beam direction. An example of a transducer element 400 operating in the transverse mode is provided in FIG. 4. In this figure, the shaded side face 404 and the face opposite (not shown in figure) to the side face 404 of the active element 402 act as electrodes. A heavy tail mass is used as the backing material 406 which helps to project the acoustic power towards the front direction. In such a design, the active element 402 in the activated direction is typically of quarter-wavelength (λc/4) in dimension. The relevant sound velocity in the active element 402 is along the activated transverse (1−) direction, designated as v11E (or v22E) where the subscripts “1” (or “2”) indicate the activated direction being 1− (or 2−) of the two possible transverse directions and the superscript “E” indicates that the active element 402 is under a state of constant electrical field in this case.
In the design of transducers operating in either the longitudinal mode or the transverse mode described above, the four side faces of the active element 302 or 402 may be shielded from the propagation medium with a soft and high-damping material to ensure that the active element is not unduly constrained laterally and free to vibrate or resonate in the intended mode and that sound wave propagation in the intended acoustic direction is realized. The stress/pressure release materials, housing as well as the lead wires connected to the electroded faces, are not shown in FIGS. 3 and 4 for clarity sake.
Polycrystalline PZT ceramics and their doped derivatives, the most popular active material for ultrasonic transducers and arrays as of to-date, have much superior longitudinal piezoelectric properties (d33 and k33 values) than transverse properties (d31 and k31 values) where d and k are the piezoelectric strain coefficient and the electromechanical coupling factor respectively; the first subscript 3 indicates that the applied electric field is in the “3”− or poling direction and the second subscript indicates the activated direction −“3” for longitudinal actuation and “1” or “2” for transverse actuation. For instance, for typical PZT ceramics, while d33=300-600 pC/N and k33=0.6-0.7 for the longitudinal mode, d31=150-300 pC/N and k31=0.34-0.40 for the transverse mode. Compared with the longitudinal properties, the transverse properties are not as favorable for efficient sound and ultrasound generation. Also, the sound velocities in both the longitudinal and transverse directions (of >2000 m/s typically) are much higher than velocities of sounds in water and human tissues. The high sound velocities and inferior transverse properties of PZT-based ceramics make longitudinal mode transducers and arrays much more popular than the transverse mode ones for sound and ultrasound generation for underwater and/or medical applications.
Another reason for the low popularity of transverse mode transducer arrays notably in the medical field is the difficulty involved in fabricating such an array by the established automatic dicing operation. The deposition of electrodes on both transverse faces and their wiring are problematic for such an array configuration.